Getting it right with a turnkey aeration system

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The Valenton wastewater treatment plant in Paris is the second biggest plant in France. As part of the refurbishment of the four process tanks, Grundfos was asked to design and install the aeration systems. This article describes how diverse challenges were resolved in both the design and installation phases.

In the design phase, special attention needed to be given to issues such as oxygenation specifications at different water temperatures, compressed air temperature and condensation in the pipework. Moreover, a success criteria for the project was to obtain a high oxygen transfer efficiency (OTE) and thereby energy efficiency. The solutions used are explained in more detail below.

From preassembled units to turn-key installation

With only a month to complete each phase of the refurbishment project, the challenge was to put two lots of 13,000 disc diffusers and pipework together into a turnkey aeration system. This was only possible because Grundfos preassembles its diffuser systems, delivering them to the site in carefully numbered crates and boxes, itemised in detail. This turned the relatively complex and time-consuming onsite installation into a comparatively simple matter of fitting the pieces together like a kitset.

High operating efficiency

A central parameter for comparing aeration systems is standard aeration efficiency (SAE), defined as the rate of oxygen transferred to the liquid per unit of power input (kgO2/kWh). SAE is dependent on a complex interplay between the system itself and conditions in and around the basin.

For the designer there are number of parameters that can be adjusted to ensure optimal aeration. Since the Valenton project was a refurbishment of existing basins, diffuser submergence and basin type were given from the outset. The main variables that could be adjusted to ensure high oxygen transfer efficiency were:

Bubble size

Unit airflow

Diffuser density

Bubble size

The key to efficient oxygen transfer is the ascent velocity of the air bubbles and air liquid interface. This dictates the time and space available for oxygen to be transferred from the bubble to the surrounding liquid.

Since oxygen transfer is the objective (some aeration systems can also be designed for mixing purposes), the refurbished system at Valenton is based on fine-bubble diffusers, as was the old system. Bubble size has a significant effect on oxygen transfer where the air/liquid interface (A/V) of the air bubble directly influences the oxygen transfer rate. The A/V ratio and thus the oxygen transfer rate can be increased effectively by decreasing bubble size. Furthermore, fine bubbles ascend slower, extending the time available for oxygen transfer.

Airflow

Ensuring sufficient airflow is fundamental to the oxygenation process. However, simply increasing the airflow to add more oxygen will have a negative impact on efficiency. Firstly, the higher the airflow, the bigger the blower (kWh). In addition, the total headloss is increased because the counter pressure from the membrane increases with higher airflow, further increasing the blower’s power consumption.

Secondly, standard oxygen transfer efficiency (SOTE) decreases as the airflow per diffuser increases (Figure 1), a high airflow rate per diffuser will thereby increase operating costs directly. The decrease in SOTE is due to the fact that:

water circulation in the aeration tank is increased, reducing retention time of air bubbles in the liquid

air bubbles produced by the diffuser will increase in diameter, reducing oxygen transfer

Once again to achieve high aeration efficiency, Grundfos aimed for a high diffuser density (about 18%) which gives a lower airflow per diffuser. The greater number of diffusers gives a higher initial cost but this is quickly offset by lower operation costs. Increasing diffuser density beyond 20% will have the opposite effect on efficiency because the bubbles begin to coalesce, creating larger bubbles and reducing the air/liquid interface.

Getting the materials right

Generally speaking, the temperature of compressed air in such a system will increase by about 10°C/mWC of headloss. This is a factor of such things as submergence depth, pipework/fittings and counter pressure at the diffuser membrane. With a submergence depth of 7.5 m at Valenton, the pressure to overcome headloss in the aeration system could push the air temperature (at the compressor) up to about 140°C in the summer.

The comparatively high temperatures meant that the piping in the basin needed to be in more heat-resistant PP instead of the more commonly used PVC. PP increases the cost of the pipework compared with PVC, but is equally durable and considerably more economical than stainless steel.

Dealing with condensation

The hot air in the system condenses readily in the submerged aeration grid, with water collecting at the lowest points. Such a build-up of water in the system reduces pipe diameter, increasing headloss and thereby operating costs.

To relieve the aeration system for condensate build-up, a purge system is incorporated at the lowest points in the aeration grid. By ensuring a lower headloss through the purge system compared with the diffusers, an airlift function is created, forcing the condensed water from the grid.

The discharge points of the purge system can either be above water in a manual system (tap) or at the bottom of the basin in a continuous, automatic purge system. While the manual system is quite time consuming, the Valenton plant required a combination of both for extra assurance that the condensed water was being removed.